To cope with the global demand for energy efficiencies in the transportation industry, various approaches have been employed in order to vary characteristics of pneumatic tires under different operating conditions (i.e., reduction of the rolling resistance of a pneumatic tire in an attempt to decrease the specific rate of hydrocarbon-based fuel consumption of the corresponding automobile or truck). The rolling resistance of a tire can effectively be reduced by diminishing the internal friction loss caused by a deformation of a tire during running. A known measure for diminishing the internal friction loss is to use higher density rubber materials in tire construction having a characteristic of a smaller internal friction loss comprising the material of the cap tread of the tire.
The low level of the internal friction loss of the above mentioned rubber material, however, seriously affects general characteristics of the tire such as braking performance, steering performance, comfort, wear-resistant properties and so forth. For instance, if a more dense rubber having a small internal friction loss is used to reduce the rolling resistance of the tire during running, various unfavorable phenomena are caused during high-speed cruising on a wet road, such as an increase of braking distance, decrease in aquaplaning, deterioration of control, degraded comfort, reduced wear, and so on. These characteristics are typically undesirable for a pneumatic tire.
Conventionally, a cap tread of a pneumatic tire has been constituted by a rubber compound which inherently has a large internal friction loss in order to improve the above-mentioned characteristics of the pneumatic tire, such as braking distance. The attempt to reduce the rolling resistance and aquaplaning of the pneumatic tire by diminishing the internal friction loss, therefore, is inevitably accompanied by a certain degradation of desirable tire characteristics. Henceforth, there has existed a conflicting design tradeoff in the conventional construction and design of a pneumatic tire that limits both the performance characteristics and the extent of fuel efficiency.
One conventional pneumatic tire attempts to supercede these design limitations by providing a moveable tread portion to produce a variable ground contact footprint. This conventional pneumatic tire claims to improve wet grip performance without affecting dry grip performance, while also decreasing rolling resistance.
More specifically, this conventional pneumatic has two tread portions. One tread portion tread is constructed to remain in contact with a road surface and the other is constructed to optionally be in contact with the road surface or withdrawn from contact with the road surface before, during or after operation. The non-moveable tread portion is located near each shoulder of the pneumatic tire. The moveable tread portion is centered near the equatorial plane of the pneumatic tire and is selectively moveable in a radial direction of the pneumatic tire toward or away from contact with the road surface.
The moveable tread portion is constructed of a rubber material having relatively high friction loss and low elastic modulus. When the moveable tread portion is in contact with the road surface, rolling resistance is increased, but braking distance is decreased. The moveable tread portion of this conventional pneumatic tire has expandable voids located underneath the tread surface and within the crown of the pneumatic tire. These voids are in communication with sources of a pressure source and can be made to increase or decrease in volume. When pressurized, the voids expand and the moveable tread portion contacts the road surface. When depressurized, the voids contract and the moveable tread portion withdraws from contact with the road surface.
When low rolling resistance is desired and braking is not anticipated, the moveable portion of the tread is withdrawn from the road surface. The moveable tread portion is normally withdrawn from contact with the road surface when the vehicle is cruising at highway speeds and low rolling resistance is desired. In this condition, the weight of the vehicle is placed on the now smaller ground-contact patch of the footprint surface area. Thus, the ground-contact footprint offers less contact friction and the average rolling resistance is reduced. Also, the tendency of the pneumatic tire to aquaplane is reduced.
When acceleration or deceleration (i.e., braking) is desired or anticipated, the voids of the conventional pneumatic tire are expanded and the moveable tread portion contacts the road surface. The friction offered by the larger ground-contact footprint creates additional braking/traction capability. A pneumatic tire having dual modes of enhanced traction and decreased aquaplaning, without requiring a costly dedicated and complex tire construction such as the conventional pneumatic tire described above, would be desirable.